MIMO Background
Multiple Input Multiple Output (MIMO) is an advanced antenna technique to improve the spectral efficiency and thereby boost the overall system communication capacity. The MIMO technique uses a commonly known notation (M×N) to represent MIMO configuration in terms number of transmit antennas (M) and receive antennas (N). The common MIMO configurations used or currently discussed for various technologies are: (2×1), (1×2), (2×2), (4×2), (8×2) and (2×4), (4×4), (8×4). The configurations represented by (2×1) and (1×2) are special cases of MIMO.
Various embodiments are described herein the context of NodeBs, eNobe Bs, and UEs, this terminology is used herein a non-limiting example manner and does not imply a certain hierarchical relation between the two; in general “NodeB” could be considered as device 1 and “UE” device 2, and these two devices communicate with each other over some radio channel. Although various embodiments are explained in the context of downlink wireless transmissions, the embodiments are also applicable to uplink wireless transmissions.
It is well known that MIMO systems can significantly increase the data carrying capacity of wireless systems. For these reasons, MIMO is an integral part of the 3rd and 4th generation wireless systems. However, for such systems, the optimal maximum-likelihood or Maximum A posteriori Probability (ML/MAP) detection for minimizing the packet error rate using exhaustive search can be impossible or not feasible to implement. This is because a MIMO detector's complexity increases exponentially with the number of transmit antennas or/and the number of bits per constellation point.
Several suboptimal detector structures have been proposed in literature for reducing the complexity of the MIMO detector. These can be classified into linear and nonlinear detectors. Linear detectors include zero-forcing (ZF) and minimum mean-square error (MMSE) detectors, and the nonlinear receivers include decision feedback, nulling-cancelling and variants relying on successive interference cancellation (SIC). These suboptimal detectors are easier to implement but their packet error rate performance is significantly inferior to that of the optimum MIMO detector. This is because most of these sub optimal detection techniques proposed in literature for cancelling multi antenna interference are proposed with/without channel coding and without utilizing the potential of cyclic redundancy check (CRC). However, in a practical system such as LTE/LTE-Advanced, Wimax, HSDPA etc., the CRC bits are appended before the channel encoder at the transmitter and the check has been done after the channel decoder to know whether the packet is received correctly or not.
Interference Cancellation when CRC is Appended
FIG. 1 shows the transmission side of a MIMO communication system with Nt transmit antennas. There are Nt transport blocks. CRC bits are added to each transport block and passed to the channel encoder. The channel encoder adds parity bits to protect the data. Then the stream is passed through an interleaver. The interleaver size is adaptively controlled by puncturing to increase the data rate. The adaptation is done by using the information from the feedback channel, for example channel state information sent by the receiver. The interleaved data is passed through a symbol mapper (modulator). The symbol mapper is also controlled by the adaptive controller. The streams output from modulator are passed through a layer mapper and a precoder. The resultant streams are then passed through IFFT block. The IFFT block is necessary or at least beneficial for some communication systems which implement OFDMA as the access technology, e.g., LTE/LTE-A, Wi-max. For other systems which implement CDMA as the access technology, e.g., HSDPA, the IFFT block is replaced by a spreading/scrambling block. The encoded stream is then transmitted through the respective antenna.
FIG. 2 shows a receiver for a multiple codeword MIMO system without interference cancellation. After the FFT operation, a MIMO detector is used to remove the multi antenna interference. A de-mapper is used to compute bit log likelihood ratios from the MIMO detector output which is in the symbol domain. The bit stream is then de-interleaved by a de-interleaver and passed to a channel decoder for decoding. A CRC check is performed on the output of the channel decoder. If the CRC check passes then transport block is considered to be passed and an ACK is sent back to the transmitter via a feedback channel (also called uplink control channel, HS-DPCCH in HSDPA, and PUCCH/PUSCH in LTE/LTE-A). In contrast, if the CRC check fails then a NAK is sent back to the transmitter using the uplink control channel.
FIG. 3 shows a schematic diagram of a MIMO receiver with an interference cancellation circuit, which concurrently decodes all the receiver codewords. Once the CRC check is made on all the codewords, the codewords whose CRC check is a pass are reconstructed and subtracted from the received signal and only those codewords whose CRC check is a fail are decoded. This process is repeated until all the codewords pass the CRC check, all the codewords fail the CRC check, or a pre-determined number of iterations is reached.
System Level Gains with Interference Cancellation
Table 1 shows example system-level simulation results in the context of 3GPP LTE downlink with 2 and 4 transmit antennas with 2 and 4 multiple code words respectively. These results assume a frequency reuse of one. Synchronous and non-adaptive HARQ with maximum of 4 retransmissions is assumed. Simulations assume Typical Urban (TU) channel model-A with 6 multipath components.
TABLE 1Relative gains for the Interference Cancellation (IC)SectorCell edgeThroughputThroughput% gain of ICConfig-(Mbps)(Kbps)SectorCell edgeurationNo ICICNo ICICThroughputThroughput2 × 214.5518.0532933824.052.754 × 420.4626.5224237529.6254.96Link Level Gains with Interference Cancellation
FIGS. 4-9 shows example link level gains achieved with interference cancellation for different transmission modes. FIG. 4 illustrates graphs of throughput of a single cell scenario under Extended Vehicular A (EVA) EVA70 medium correlation with Fixed Reference Channel (FRC) TM3. FIG. 5 illustrates graphs of throughput of a single cell scenario under EVA5 medium correlation with FRC TM4. FIG. 6 illustrates graphs of throughput of a single cell scenario under Extended Pedestrian A (EPA) EPA5 medium correlation with FRC TM9. FIG. 7 illustrates graphs of throughput of a multi-cell scenario under EVA70 medium correlation with FRC TM3. FIG. 8 illustrates graphs of throughput of a multi-cell scenario under EVA5 medium correlation with FRC TM4. FIG. 9 illustrates graphs of throughput of a multi-cell scenario under EVA5 medium correlation with FRC TM9.
It can be observed that significant gains can be achieved in almost all cases simulated, including single cell scenarios and multi-cell scenarios. Two types of interference cancellation receivers are considered, namely symbol level interference cancellation (SLIC) 404, 504, 604, 704, 804, 904 and codeword level interference cancellation (CWIC) 402, 502, 602, 702, 802, 902 receivers. A SLIC type of receiver cancels the interference based on a modulation symbol level, utilizing successive application of linear detection, reconstruction, and cancellation. A CWIC type of receiver cancels the interference based on a decoded codeword level which is taken as a non-linear receiver that decodes and subtracts the interference. CWIC requires more computations and is more complex to implement. The performance of CWIC is better than SLIC.
Uplink Feedback Channel
FIG. 10 shows the uplink feedback channel structure in HSPA/LTE. Where the Hybrid Automatic Repeat Request—Acknowledgement (HARQ-ACK) is transmitted corresponding to the data traffic channel and the channel state information (CSI) is transmitted in the next slot. Note that CSI may consist of channel quality indicator (CQI), precoding matrix index (PMI)/Precoding control index (PCI), rank information (RI)/number of transport blocks preferred (NTBP) and preferred sub band indices.
Problems with Existing Solutions
When the UE performs interference cancellation (IC), the network node (e.g. Node B in HSPA or eNode B in LTE) does not know whether the UE is able to suppress the inter-stream interference at any instance. For example the UE may switch off the IC or reduce the IC capacity (partial IC) in some scenarios. In these cases, since the network node does not know the status of UE IC capability the radio resources at the network node may be wasted or are underutilized.
The approaches described in the Background section could be pursued, but are not necessarily approaches that have been previously conceived or pursued. Therefore, unless otherwise indicated herein, the approaches described in the Background section are not prior art to the claims in this application and are not admitted to be prior art by inclusion in the Background section.